Electrochemical treatment of metal surfaces



United States Patent 3,337,431 ELECTROCHEMICAL TREATMENT OF METAL SURFACES Yoichi Kitamura, Yokohama, Tsuneo Inui, Hikari-shi,

Yamaguchi, and Etsuro Shuto, Yokohama, Japan, assignors to Toyo Kohan Co., Ltd., Tokyo, Japan, a corporation of Japan No Drawing. Filed Oct. 30, 1963, Ser. No. 319,952 Claims priority, application Japan, Nov. 10, 1962, 37/49,394 6 Claims. (Cl. 204-56) The present invention relates to a process of electrochemical treatment to form a protective coating on surfaces of iron, steel, aluminum and aluminum alloys, and to electrolytes to be employed therein. More particularly the present invention relates to a process of forming a hydrated chromium oxide coating which has high corrison resistance and provides an excellent base for the adhesion of paint, varnish, lacquer and other organic finishes, on surfaces of iron, steel, aluminum and aluminum alloys in a chromic acid solution. conventionally a number of similar electrolytic methods have been reported. For example, the following electrolytes have been proposed:

(1) Aqueous solution of chromic acid to which boric acid or a salt thereof is added (U.S. Patent No. 2,733,199, U. S. Patent No. 2,780,592),

(2) Aqueous solution of chromic acid to which phosphoric acid is added (U.S. Patent No. 2,769,774),

(3) Aqueous solution of chromic acid in which the ratio of hexavalent chromium ion to trivalent chromium ion is kept within the range of 1 to (Japanese Patent No. 272,741),

(4) Aqueous solution of chromic acid to which trivalent chromium ion, phosphoric acid and boric acid are added (U.S. Patent No. 3,032,487),

(5) Aqueous solution of chromic acid to which disulfonic acid or a salt thereof derived from phenol or catechol is added (U.S. Patent No. 2,998,361),

(6) Aqueous solution of chromic acid alone (U.S. Patent No. 3,081,238).

It is an object of the present invention to provide a method of forming in a very short time an extremely thin coating with excellent corrosion resistance and lacquer adhesion qualities. It is another object of the present invention to apply the coating onmetallic sheets, hoops, strips, wires, tubes and mechanically formed articles.

A further object of the present invention is to provide metal surfaces which are better bases for the adhesion of organic coatings than in conventional electrolytic tinplate, even when they are mechanically formed and exposed to various conditions after coating.

It is a still further object of the present invention to provide an electrolytic bath having better throwing power and higher current efliciency and to allow more stable operation within a broader concentration range, in comparison with those of the conventional processes mentioned above.

According to the present invention, the electrolytic bath is an aqueous solution consisting essentially of chromic acid, trivalent chromium ion in a definite proportion to hexavalent chominm ion and sulfuric acid in the presence of a fluorine compound, wherein the anode is lea-d or lead alloy and the cathode is one of the above mentioned metals to be coated. In accordance with the present invention, a hydrated oxide film of chromium is formed on the metal surfaces.

As the metals to be treated according to the present invention, pure iron, low carbon steels, pure aluminum and its alloys containing more than 90% of aluminum are the most suitable. As the anode metals, pure lead,

lead-tin alloys, lead-antimony alloys and lead-silver alloys are suitable.

Prior to the treatment of the present invention the surface of metallic material should be cleaned by any conventional treatment such as degreasing with alkalis and pickling with acids for iron and steel, and cleaning with alkalis for aluminum and aluminum alloys.

The electrolyte employed in the present invention contains chromium ion which is mainly supplied by chromic acid.

The optimum concentration of chromic acid is within the range from 40 g./l. to 100 g./l. It the concentration of chromic acid is below 40 g./l., the film formed has inferior formability, decreased lacquer adhesion quality and weaker chemical resistance, and it the concentration 7 of chromic acid is above 100 g./l., the film is transformed from the state of hydrated chromium oxide to the state of metallic chromium with poorer corrison resistance.

Chromium ions existing in the electrolyte are the trivalent chromium ion and hexavalent chromium ion.

In general, the optimum Weight ratio of hexavalent chromium ion to trivalent chromium ion ranges from 20 to 150, and the concentration of trivalent chromium ion should not exceed 2.5 g./l. If the Cr /Cr ratio becomes larger than 150, current efficiency goes down, the film formed becomes uneven and non-uniform. On the other hand, if the Cr /Cr ratio is smaller than 20, or trivalent chromium ion concentration exceeds 2.5 g./ 1., conductivity of the electrolyte goes down, bath voltage rises, current efliciency drops and, furthermore, formability of the film formed is lessened.

The present electrolyte initially contains reducing agents which serve to maintain the chromium ion in trivalent form in aqueous solution, thereby enabling continual operation.

The reducing agent must be a water soluble organic compound of simple structure, having alcoholic hydroxyl or phenolic hydroxyl groups. These hydroxyl groups are readily oxidized and readily decomposed or evaporated in a hot aqueous solution of chromic acid without leaving a precipitate. Heating an aqueous solution of chromic acid to which a reducing agent has been added for about 30 minutes at 60 0., provides the necessary amount of trivalent chromium ion.

The reducing agents include water soluble alcohols such as methyl alcohol, ethyl alcohol, propyl alcohol and monoethanolamine; polyalcohols, such as ethylene glycol, propylene glycol, glycerol, diethanolamine, triethanolamine, pentaerythritol, sorbitol and mannitol; phenol; and phenol sulfonic acids such as phenol-2,4-disulfonic acid,

, catechol-3,5-disulfonic acid, phenol-2,4,6-trisulfonic acid,

2-naphthol-3,6-disulfonic acid, and 1,8-dihydroxynaphthalene-3,6-disulfonic acid. Cresol, xylenol and polyphenols (catechol, hydroquinone) are not desirable, because they form a precipitate in the aqueous solutions of chromic acid. The reducing agent may be present in such an amount that the required concentration of trivalent chromium ion is produced in the aqueous solution of chromic acid.

Furthermore, the bath of the present invention must contain sulfuric acid. The sulfuric acid concentration in aqueous solutions of chromic acid should be in the range between 0.1 g./l. and 0.5 g./l. If the sulfuric acid concentration is lower than 0.1 g./l., it is hard to get a uniform film and to prevent yellow stains on the film which have a detrimental effect on appearance. As the sulfuric acid concentration increases, current efiiciency gradually goes up, but adhesion of the film formed to the base metal, formability and corrosion resistance of the film are decreased. Therefore the sulfuric acid concentration should not exceed 0.5 g./ 1. Accordingly, the sulfuric acid concentration should be maintained as low as possible within the range between 0.1 g./l. and 0.5 g./l. In case phenol sulfonic acids are employed as reducing agents, it is not necessary to conduct further addition of sulfuric acid, so far as the required concentration of sulfuric acid is obtained with the phenol sulfonic acids.

It is quite easy to form a thin film of hydrated chromium oxide on the surface of cathode metal, by means of applying direct current electrolysis in the bath mentioned above, composed of chromic acid, trivalent chromium ion and sulfuric acid, in which the said chemicals are admixed in concentrations mentioned above, the cathode is the metal to be treated and the anode is lead or lead alloy.

Electrolytes of the present invention may further contain fluorides, acidic fluorides, fiuosilicates and fluoborates. The fluorine compounds help to obtain higher current efliciency and better throwing power and more excellent stabilized films in a wider concentration range.

Typical examples of these compounds are as follows:

Hydrofluoric acid; fluorides, such as sodium fluoride, potassium fluoride, ammonium fluoride and acidic fluorides, such as acidic sodium fluoride, acidic potassium fluoride, acidic ammonium fluoride, and hydrofluosilicic acid; fluosilicates, such as sodium fluosilicate, potassium fluosilicate, ammonium fluosilicate, and hyd-rofluoboric acid: fluoborates, such as sodium fiuoborate, potassium fluoborate, and ammonium fluoborate.

The amount of addition of these fluorine compounds should not exceed 1.0 g./l. When the amount exceeds the above range, formability, lacquer adhesion quality and corrosion resistance of the film formed are badly affected.

The conditions for the electrolysis of the present in vention are preferably as follows, namely, temperature of treatment; 40-70 C., current density; not less than 12.5 a./ sq. dm., time for treatment; l20 seconds.

An increase in temperature results in a decrease in current efficiency and accelerates the transformation of the film from hydrated chromium oxide to metallic chromium which leads to poorer corrosion resistance. In actual operations, the increase in temperature will cause an increase in evaporation loss of water and make it diflicult to control the concentration of the electrolytic bath. Operations should be controlled at temperatures below 70 C., if possible. On the other hand, as the decreases in temperature results in the decrease in formability of the film formed, care should be taken not to lower the temperature below 40 C. Therefore, it is preferable to keep the treating temperature within the range between 40 C. and 70 C.

It is desirable to increase current density as high as possible. Operation with higher current density for a shorter time may give better results in formability and corrosion resistance of the film. A current density of not less than 12.5 a./sq. dm. is necessary, and is preferably not more than 40 a./sq. dm. If current density is lower than 12.5 a./sq. dm., corrosion resistance will not be imroved very much even if a longer treating time is employed.

Treating time depends on the thickness of the film required if the other conditions are settled. In general, however, treatment for 1 to 20 seconds may be suflicient and treatment for 6 to 9 seconds may readily give a film of about 2 milligrams per sq. dm.

In the case of application of the treatment described in the present invention to iron and steel, the gradual color changes of the film according to conditions for the electrolyte and electrolysis are characteristic, by which one can understand the properties of the film formed to some extent. The color changes are: blue bluish purple bluish white, as chromic acid concentration increases; bluish yellowbluish purpleebluish white, as trivalent chromium ion concentration increases; bluish purple bluish yellowyellow, as sulfuric acid concentration increases; blue bluish purplee bluish white, as a fluorine compound concentration increases; yellowbluish yellow+blue bluish purple, as temperature rises; bluish yellow bluebluish purple, as current density increases; and bluish purpleblue bluish yellow yellow as the amount of treating current increases. For the purpose of obtaining a bluish purple film of the desirable properties, it is preferable to employ medium concentration of chromic acid and trivalent chromium ion, lower sulfuric acid concentration, medium concentration of a fluorine compound, higher temperature and current density and smaller amount of treating current or shorter treating time. Unlike iron and steel, a colorless or grayish white film is almost always formed in the case of application of the treatment described in the present invention to aluminum and aluminum alloys.

The iron and steel treated according to the present invention can withstand an outdoor exposure test for one month and also resist a salt spray test for 24 hours without any signs of rust, even if the thickness of the treated film is as thin as 0.05 micron. After applying a film of about 10 microns of modified alkyd or modified epoxy lacquer on the steel plate treated according to the present invention, the lacquered plate can be deeply drawn with a drawing ratio of 2.2 and when a piece of adhesive tape is applied firmly on the lacquered side of the deeply drawn cup and then pulled off quickly, the tape carries away no lacquer. Furthermore, the lacquer film adheres firmly on the cup, even when it is tested with the adhesive tape after the cup is immersed in boiling water for one hour. Even after the lacquer film of the lacquered plate itself is cross-hatched and the plate is immersed in boiling water for eight hours, the film can never be peeled off by using the adhesive tape. In electrolytic tinplate, 70% of the lacquer film is stripped under similar conditions after boiling for one hour.

Since the surface film which has been made according to the process described in the present invention adheres very firmly to base metals, it cannot be peeled off by any ordinary mechanical means such as pulling, bending and deep drawing.

Furthermore, the surface film formed according to the present invention easily withstands alkalis, salts, petroleum oils, fats and organic solvents, excepting strong acids. The steel plate treated according to the present invention can be used as a material for cans and containers for detergents, soaps, industrial chemicals, petroleum oils and paints in place of tinplate which is susceptible to alkalis and salts.

As described above, metal articles treated according to the present invention have excellent lacquer adhesion quality as well as excellent corrosion resistance, heat resistance up to 300 C., and weldability as good as base metals, even if the applied film is extremely thin.

Example 1 A 0.25 mm. cold rolled low carbon steel sheet, so-called black plate, was cathodically cleaned for 20 seconds at a current density of 4 a./sq. dm. at C. in a 7% sodium hydroxide solution, then rinsed with water, pickled for 10 seconds at room temperatures in 7% sulfuric acid and again rinsed with water. Then, the sheet was put in an electrolytic bath and direct current was passed, wherein the sheet acted as a cathode with lead-antimony (:5) as an anode. The composition of the bath was 40 g./l. of chromic acid, 0.50 g./l. of ethyl alcohol, 0.62 g./l. of trivalent chromium ion, 0.10 g./l. of sulfuric acid, 0.05 g./l. of ammonium fluosilicate and the Cr /Cr ratio was 34. The temperature was 50 0, current density was 15 a./ sq. dm., and time for treatment was 10 seconds. The electrolytic treatment of the steel sheet was conducted in the electrolyte obtained by adding 0.5 g./l. of ethyl alcohol as a reducing agent to an aqueous solution of 40 g./l. chromic acid, heating the mixture for 30 minutes at 60 C. to produce 0.62 g./l. of trivalent chromium ion, and thereto adding 0.1 g./l. of sulfuric acid and then 0.05 g./l.

of ammonium fluosilicate. The film formed was blue and transparent and the steel sheet thus treated showed a few rusty spots after being subjected to a salt spray test with a 5% sodium chloride solution for 20 hours at 35 C. When the steel sheet was coated with about 8 microns of melamine modified alkyd white enamel, and deeply drawn to the cup with a drawing ratio of 2.0, no adhesion loss of the enamel was found on the deeply drawn part of the cup.

In Examples 2 to 8 the electrolyte was prepared by adding chemicals in same manner as in Example 1.

Example 2 The same kind of cold rolled low carbon steel sheet was subjected to the same pretreatments as described in Example 1, then the sheet was put in an electrolytic bath and direct current was passed, wherein the sheet acted as a cathode with lead-antimony (95:5) as an anode. The composition of the bath was 50 g./l. of chromic acid, 0.20 g./l. of monoethanolamine, 0.51 g./l. of trivalent chromium ion, 0.20 g./l. of sulfuric acid, 0.20 g./l. of ammonium fluoborate and the Cr /Cr ratio was 51. The temperature was 60 C., current density was 23 a./ sq. dm., and time for treatment was seconds.

The film formed was bluish purple and transparent, and the steel sheet thus treated showed only few signs of rust after being subjected to a salt spray test with a 5% sodium chloride solution for 24 hours at 35 C. The lacquer adhesion quality after mechanical forming was the same as in Example 1.

Example 3 The same kind of cold rolled low carbon steel sheet was subjected to the same pretreatments as described in Example 1. Then the sheet was put in an electrolytic bath and direct current was passed, wherein the sheet acted as a cathode with lead-antimony (95:5) as an anode. The composition of the bath was 50 g./l. of chromic acid, 0.50 g./l. of phenol-2,4-disulfonic acid, 0.30 g./l. of trivalent chromium ion, and 0.60 g./l. of hydrofluosilicic acid and the Cr /Cr ratio was 87. The temperature was 60 C., current density was 20 a./sq. dm. and time for treatment was 1 5 seconds.

The film formed was bluish white, and the steel sheet thus treated had a similar corrosion resistance and lacquer adhesion quality as those in Example 2.

Example 4 A 0.4 mm. cold rolled steel sheet was subjected to the same pretreatments as described in Example 1. Then the sheet was put in an electrolytic bath and direct current was passed, wherein the sheet acted as a cathode with lead-antimony (95 :5) as an anode. The composition of the bath was 60 g./l. of chromic acid, 0.50 g./l. of phenol, 0.38 g./l. trivalent chromium ion, 0.50 g./l. of sulfuric acid, and 0.50 g./l. of sodium fluoride and the Cr /Cr ratio was 82. The temperature was 45 C., current density was 12.5 a./sq. dm. and time for treatment was 10 seconds. The film formed was bluish purple and the treated steel sheet had properties similar to those in Example 2.

Example 5 A 0.5 mm. aluminum sheet (99.0%) was cleaned for 10 seconds at room temperature in a 4% sodium bicarbonate solution, rinsed with water and then the sheet was put in an electrolytic bath and direct current was passed, wherein the sheet acted as a cathode with lead-antimony (95:5 as an anode. The composition of the bath was 50 g./l. of chromic acid, 0.15 g./l. of ethyl alcohol, 0.20 g./l. of trivalent chromium ion, 0.20 g./l. of sulfuric acid, and 1.0 g./l. of ammonium fiuosilicate. The Cr-'/Cr ratio was 130. The temperature was 40 C., current density was 12.5 a./sq. dm. and time for treatment was 20 seconds. The film formed was bright grayish white, and the aluminum sheet treated showed no white rust at all, even Example 6 The same kind of cold rolled steel sheet was subjected to the same pretreatments as in Example 4, and then the sheet was put in an electrolytic bath and direct current was passed, wherein the sheet acted as a cathode with lead-antimony (:5) as an anode. The composition of bath was 50 g./l. of chromic acid, 0 .20 -g./l. of methyl alcohol, 0.40 -g./1. of trivalent chromium ion, 0.20 g./l. of sulfuric acid, and 0.25 g./l. of hydrofluoboric acid. The Cr /Cr ratio was 65. The temperature was 55 C., current density was 20 a./sq. dm. and time for treatment was 10 seconds. The film formed was bluish purple and transparent, and the steel sheet treated showed a few rusty spots after being subjected to the same salt spray test as in Example 1 for 24 hours. When the steel sheet was coated in the same manner as in Example 1 and deeply drawn to the cup with a drawing ratio of 2.2, no adhesion loss of the coated enamel was found on the deeply drawn part of the cup.

Example 7 The same kind of cold rolled steel sheet was subjected to the same pretreatments as in Example 4 and then the sheet was put in an electrolytic bath and direct current Was passed, wherein the sheet acted as a cathode with lead-antimony (95 :5 as an anode. The composition of bath was 50 g./1. of chromic acid, 0.20 g./l. of ethylene glycol, 0.37 g./l. of trivalent chromium ion, 0.10 g./l. of sulfuric acid, and 0.20 g./l. of acidic ammonium fluoride. The Cr /Cr ratio was 70. The temperature was 60 C., current density was 24 a./sq. dm., and time for treatment was 10 seconds. The film formed was bluish yellow and transparent, and the treated steel sheet had A 0.8 mm. cold rolled steel sheet was subjected to the same pretreatments as in Example 1, and then the sheet was put in an electrolytic bath and direct current was passed, wherein the sheet acted as a cathode with leadantimony (95 :5 as an anode. The composition of bath was g./l. of chromic acid, 2.0 g./l.. of ethyl alcohol, 2.5 g./1. of trivalent chromium ion, 0.20 g./l. of sulfuric acid, and 0.10 g./l. hydrofluoboric acid. The Cr /Cr ratio was 21. The temperature was 70 C., current density was 20 a./sq. dm. and time for treatment was 6 seconds.

The film formed was pale bluish white, and the steel sheet treated had properties similar to those in Example 2.

What we claim is:

1. A method of forming a protective coating of hydrated chromium oxide on a metal surface, which comprises preparing an electrolytic bath by adding to an aqueous solution consisting essentially of 40-100 grams per liter of chromic acid, a water soluble organic hydroxyl compound in such a stoichiometrical amount that not more than 2.5 grams per liter of trivalent chromium ion and 20-150 parts by weight of hexavalent chromium ion per part of trivalent chromium ion are formed, (ll-0.5 gram per liter of sulfuric acid and an inorganic fluorine compound present in an amount up to 1.0 gram per liter, and effecting electrolysis in said electrolytic bath using as cathode the metal to be coated and an anode of leadbase metal.

2. A method according to claim 1 wherein the water soluble organic hydroxyl compound is selected from the group consisting of methyl alcohol, ethyl alcohol, propyl alcohol, monoethanolamine, a polyhydric alcohol, phen01, phenol-2,4-disulfonic acid, catechol-3,5-disulfonic acid, phenol-2,4,6-trisulfonic acid, 2-naphthol-3,6-disultonic acid and 1,8-dihydroXynaphthalene-3,6-disulfonic acid.

3. A method according to claim 1 wherein the metal to be coated is selected from the 'group consisting of iron, steel, aluminum and aluminum base alloys.

4. A method according to claim 1 wherein the inorganic fluorine compound is selected from the group consisting of hydrofluoric acid, hydrofluosilicic acid, hydrofluoboric acid and water soluble salts thereof.

5. A method according to claim 1 wherein the electrolysis is carried out at a temperature of about 40-70 C. and with a cathode current density of not less than 12.5 amperes per square decirneter for a period of 1-20 seconds.

6. An aqueous electrolytic bath consisting essentially of 40-100 grams per liter of chromic acid wherein chromium ion is present in such a manner that not more than 2.5 grams per liter of trivalent chromium ion and 20-150 parts by weight of hexavalent chromium ion per part of trivalent chromium ion are present, 0.1-0.5 gram per liter of sulfuric acid and an inorganic fluorine compound selected from the group consisting of hydrofluoric acid, hydrofiuosilicic acid, hy drofluoboric acid and water 8 soluble salts thereof present in an amount up to 1.0 gram per liter.

References Cited UNITED STATES PATENTS 1,545,498 7/1925 Klinger et a1. 148-6.2 2,459,365 1/1949 Coates 204--56 2,733,199 1/1956 Wick 20456 2,998,361 8/1961 Kitamura 204-56 3,011,958 12/1961 White 20456 3,032,487 5/1962 Yonezaki et a1. 204-56 3,081,238 3/1963 Gurry 20456 X 3,118,824 1/1964 Yonezaki et a1 204--56 3,138,548 6/1964 Ham et a1 204-56 X FOREIGN PATENTS 1,152,591 8/1963 Germany. 1,152,869 8/1963 Germany.

JOHN H. MACK, Primaiy Examiner.

HOWARD S. WILLIAMS, Examiner.

G. KAPLAN, Assistant Examiner. 

1. A METHOD OF FORMING A PROTECTIVE COATING OF HYDRATED CHROMIUM OXIDE ON A METAL SURFACE, WHICH COMPRISES PREPARING AN ELECTROLYTIC BATH BY ADDING TO AN AQUEOUS SOLUTION CONSISTING ESSENTIALLY OF 10-100 GRAMS PER LITER OF CHROMIC ACID, A WATER SOLUBLE ORGANIC HYDROXYL COMPOUND IN SUCH A STOICHIOMETRICAL AMOUNT THAT NOT MORE THAN 2.5 GRAMS PER LITER OF TRIVALENT CHROMIUM ION AND 20-150 PARTS BY WEIGHT OF HEXAVALENT CHROMIUM ION PER PART OF TRIVALENT CHROMIUM ION ARE FORMED, 0.1-0.5 GRAM PER LITER OF SULFURIC ACID AND AN INORGANIC FLUORINE COMPOUND PRESENT IN AN AMOUNT UP TO 1.0 GRAM PER LITER, AND EFFECTING ELECTROLYSIS IN SAID ELECTROLYTIC BATH USING AS CATHODE THE METAL TO BE COATED AN AN ANODE OF LEADBASE METAL. 